Bone marrow is a soft, gelatinous tissue in the hollow interior of bones that functions as the body’s cellular manufacturing plant. Making up about 4% of an individual’s body weight, this spongy interior houses specialized progenitor cells that continuously produce foundational cells. These cells are then distributed throughout the body to perform functions necessary for survival.
Primary Cell Types from Bone Marrow
Bone marrow houses two main categories of stem cells: hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). HSCs are the originators of every type of blood cell, including red blood cells that carry oxygen, white blood cells that form the immune system, and platelets that enable clotting.
The second type is the mesenchymal stem cell (MSC), also known as a marrow stromal cell. MSCs are multipotent and can develop into various structural tissues, forming osteoblasts (bone), chondrocytes (cartilage), and adipocytes (fat). They can also differentiate into myocytes, which are muscle cells.
These two cell populations coexist within the bone marrow. HSCs are committed to forming blood and immune cells, while MSCs generate the cells that create the body’s structural framework and connective tissues.
Natural Function and Development
Hematopoiesis is the continuous production of blood cells originating from HSCs. Because blood cells have limited lifespans—with red blood cells lasting about 120 days—HSCs must constantly divide and differentiate to replace them. This constant renewal ensures the body has a stable supply of cells for oxygen transport, immunity, and wound healing.
During times of stress, such as an infection or significant blood loss, HSCs can accelerate their production to meet the demand. This adaptive response is regulated by signals from the cell’s microenvironment, or niche, within the bone marrow.
Mesenchymal stem cells perform a parallel function in the maintenance and repair of skeletal tissues. They respond to injury by differentiating into bone, cartilage, or fat cells as needed to support tissue turnover. MSCs also reside within the bone marrow stroma, where they help regulate the activity of hematopoietic stem cells.
Therapeutic Uses in Modern Medicine
The properties of bone marrow cells are harnessed for medical treatments, particularly for diseases of the blood and immune system. The most established application is hematopoietic stem cell transplantation, also known as a bone marrow transplant. This procedure is used for conditions like leukemia, lymphoma, and aplastic anemia, where a patient’s marrow is diseased or damaged by chemotherapy.
Transplants can be autologous, using a patient’s own stored stem cells, or allogeneic, using cells from a genetically matched donor. Allogeneic transplants provide a new immune system that can help fight certain cancers but carry risks related to immune compatibility.
The therapeutic use of mesenchymal stem cells is a growing area of regenerative medicine. Clinical trials are exploring their effectiveness in treating conditions such as osteoarthritis by promoting cartilage regeneration and repairing cardiac tissue after a heart attack.
MSCs are also studied for their immunomodulatory properties, which allow them to influence the immune response. This makes them a candidate for treating autoimmune disorders and complications like graft-versus-host disease. While many MSC-based therapies are still experimental, their potential continues to drive research.
The Process of Cell Harvesting and Transplantation
One method for collecting cells is bone marrow aspiration, a surgical procedure performed under general anesthesia. In this process, doctors use a needle to withdraw liquid marrow directly from the back of a donor’s hip bones. This method collects a rich concentration of stem cells directly from their source.
A common alternative is peripheral blood stem cell (PBSC) harvesting. For several days, the donor receives medication that stimulates the bone marrow to release stem cells into the bloodstream. The collection is done through apheresis, where blood is drawn, circulated through a machine to separate the cells, and then returned to the body.
The harvested cells are given to the recipient through an intravenous infusion, similar to a blood transfusion. The patient is awake during this procedure. The infused stem cells then travel through the bloodstream to the recipient’s bone marrow.
There, they begin a process called engraftment, where they establish themselves and start to produce new, healthy blood cells. This phase marks the beginning of the patient’s recovery as their body rebuilds its blood and immune systems.